The industrial scale preparation of di- and polyisocyanates by reaction of the corresponding amines with phosgene has long been known from the prior art, and the reaction can be conducted in the gas or liquid phase and batchwise or continuously (W. Siefken, Liebigs Ann. 562, 75-106 (1949)). Processes for preparing organic isocyanates from primary amines and phosgene have already been described many times before; see, for example, Ullmanns Encyklopadie der technischen Chemie [Ullmann's Encyclopedia of Industrial Chemistry], 4th ed. (1977), volume 13, p. 351 to 353 and G. Wegener et. al. Applied Catalysis A: General 221 (2001), p. 303-335, Elsevier Science B.V. There is use here on the global scale both of aromatic isocyanates, for example methylene diphenyl diisocyanate (MMDI—“monomeric MDI”), polymethylene polyphenylene polyisocyanates (i.e. the higher homologs of MMDI, including PMDI, “polymeric MDI”; these are always obtained in industry in a mixture with MMDI components) or tolylene diisocyanate (TDI), and of aliphatic isocyanates, for example hexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI).
The industrial scale preparation of phosgene which is used in the phosgenation of the corresponding amines from CO and chlorine over activated carbon catalysts in a shell and tube reactor is likewise known from the prior art (e.g. Ullmann's Encyclopedia of Industrial Chemistry, 5th ed. Vol. A 19, p. 413ff., VCH Verlagsgesellschaft mbH, Weinheim, 1991). This involves combining carbon dioxide in a stoichiometric excess with chlorine and passing them over a fixed bed catalyst. The catalyst used for industrial purposes is activated carbon, and the selection of a suitable activated carbon is made empirically to the present day (Mitchell et al.: Selection of carbon catalysts for the industrial manufacture of phosgene; Catal. Sci. Technol., 2012, 2, 2109-2115).
WO 2009/037179 A1 is concerned with a process for preparing isocyanates with minimum stockpiling of phosgene and a minimum amount of phosgene present in each of the process stages. For this purpose, the application proposes a process in which the phosgene present in the process stages is essentially in gaseous form.
WO 2013/029918 A1 describes a process for preparing isocyanates by reacting the corresponding amines with phosgene, which can also be conducted at different plant loads without any problems; more particularly, even in the case of operation of the plant within the partial load range, the mixing and/or reaction is to be effected within the dwell time window optimized in each case, by increasing the ratio of amine to phosgene or adding one or more inert substances to the phosgene and/or amine stream. The process of the invention is said to enable operation of an existing plant at different modes with uniform product and process quality. This is intended to dispense with the procurement of multiple plants with different nameplate capacities.
The application teaches that essential parameters of a phosgenation such as, in particular, the residence times of the coreactants in the individual apparatuses are optimized for the operation of the production plant at nameplate capacity, which can lead to problems with regard to yield and product purity when the plant is run at lower than nameplate capacity (cf. page 2 lines 20 to 36). In order to be able to attain the optimized—narrow—dwell time window even at partial load (i.e. compared to operation at nameplate capacity with reduced amine flow), it is suggested that either the phosgene flow and/or the level of inerts be increased (cf. page 3 lines 5 to 19), preferably in such a way that the total flow rate of all components corresponds essentially to that at nameplate capacity (cf. page 6 lines 4 to 8). The application does mention startup and shutdown processes in the description of the background of the invention claimed on page 2, but does not disclose any technical teaching at all with regard to the specific procedure by which a production plant not in operation (i.e. with the amine flow and hence the flow of isocyanate produced equal to zero) is most advantageously brought to the desired operating state of nameplate capacity, or by which a production plant in operation is most advantageously shut down (i.e. the amine flow is reduced to zero, such that no further isocyanate can be produced). The technical measures disclosed in the application (i.e. the increase in the phosgene flow and/or the level of inerts) are to be viewed exclusively in the context of the problem of operation (i.e. the amine flow is significantly greater than zero) of a production plant at lower than nameplate capacity, or of the problem of how a plant being operated at nameplate capacity can advantageously be switched to operation at lower than nameplate capacity (see the examples).
The reaction output from the phosgenation line can, as described in EP 1 546 091 B1, be worked up. The reaction product is worked up in a laminar evaporator, preferably a falling-film evaporator, in which phosgene and HCl are gently evaporated.
U.S. Pat. No. 5,136,087 (B) likewise describes the removal of phosgene from the reaction mixture from the phosgenation by means of an inert solvent vapor that can originate from the solvent recovery of the phosgenation plant.
One possible embodiment of the solvent removal and recovery is described in EP 1 854 783 B1. Di- and polyisocyanates from the diphenylmethane series (MDI) which have been obtained by reaction of appropriate amines dissolved in a solvent with phosgene are first freed of hydrogen chloride and excess phosgene, and then a distillative separation of this crude solution into isocyanates and solvents is conducted. The solvent is recycled into the process for preparation of solutions of the feedstocks for the polyisocyanate preparation.
The quality of a process for preparing di- and polyisocyanates is defined firstly by the content of unwanted secondary components and impurities in the crude product that arise from improper running of the reaction. Secondly, the quality of a process is defined in that the overall process can be operated without technical production outage or problems that necessitate intervention into the process, and that losses of feedstocks can be avoided or at least minimized.
Such problems can arise on startup (putting the production plant into operation) or shutdown (stopping the production plant) for the phosgenation of the corresponding amines Problems of this kind may, for example, be that solids are formed, which lead to caking and blockage in the equipment (mixer, nozzle, reactor walls, conduits, etc.). A further disadvantage is that, in the event that inspection, maintenance, repair and cleaning operations are necessary on or in a reactor or another plant section, it is regularly necessary always to switch off all plant sections since the process steps build on one another and hence always proceed successively. As a result, the entire plant has to be emptied, which results in a considerable amount of reject material and is very time-consuming. Furthermore, energy has to be expended in order to bring reactors and plant sections back to the respective operating temperatures. Such production shutdowns for plant inspections, repair and cleaning measures or shortfalls of raw material or auxiliary that occur, whether planned or unplanned, are recurrent plant states which have a considerable influence on the economic operation of a plant or process that works continuously.
Although the prior art processes described are successful in preparing di- and polyisocyanates with high yield without resulting in a loss of quality in the end products, the only processes described are in the normal state of operation. Production stoppages for plant inspections, repair and cleaning measures or, for example, shortages of raw material auxiliary are not taken into account. At the same time, production shutdowns, planned or unplanned, are recurrent plant states which have a considerable influence on the economic operation of a continuously operating plant.
Such a production shutdown may be an inspection shutdown which is planned in advance, for which purpose the plant is run down, the energy supplies are switched off and typically all plant sections that are to be inspected are opened and cleaned for the purpose of examination. Such an inspection may take one or more weeks. After the inspection has ended, the production plant is closed, inertized if necessary, provided with auxiliaries and, once the appropriate forms of energy and raw materials are available, started up again. However, a production shutdown is not necessarily associated with opening or any other mechanical intervention into a reactor or another apparatus in the plant, but may also be connected to the shutdown and restarting of the production plant for various other reasons, as, for example, in the event of a failure in the raw material supply. In such a case, the plant is typically run in part-load operation and, in the worst case, when the logistical supply chain is interrupted, has to be shut down. Furthermore, production shutdowns may be forced by requirements for maintenance, cleaning or repair in the production plant. Shutdowns here in the preparation process for di- and polyisocyanates are typically described as short when production is stopped for up to one day. It is a feature of all these production shutdowns in practice that there are losses of production, and that, on restarting of the plant, for example when inertization is necessary, nitrogen is consumed or, in the heating of the plant or the feedstocks, forms of energy such as steam and power are required.
The person skilled in the art is aware that an industrial process operated semicontinuously or continuously proceeding from a production plant in operation cannot be switched instantaneously to a production shutdown, but has to be run down in a controlled manner beforehand. This is also the case for a plant outage in the event of an accident. In order to be able to produce again after the production shutdown, the plant has to be run back up to the process parameters prior to the production shutdown. Reactants and apparatuses have to be heated up, apparatuses may have to be inertized, and the loading of the apparatuses with the reactants is gradually increased to the desired target value. During this startup phase, there is thus still loss of production volume, and a disproportionate amount of energy has to be expended in order to prepare the cooled plant for startup and then to run it up to the desired target value with observation of all operationally relevant parameters as well.
What would thus be desirable would be a process in which simple measures enable optimization of production shutdowns in the operation of the preparation process for di- and polyisocyanates in terms of time taken, energy consumption, auxiliary and raw material consumption and/or reduction in wastes. This would lead to a not inconsiderable degree of improvement in productivity or economic viability of a continuously operated process or a corresponding production plant.